The present invention relates to a sheet feeding apparatus in a printer, and more particularly to a sheet feeding apparatus which can properly feed both a continuous form and cut sheets.
Many conventional printers are provided with paper feeding rollers, which are driven by a drive motor, at upstream and downstream sides of the platen in a paper feed direction. In order to prevent friction on the platen, the platen and these paper feeding rollers are engaged with each other by way of a series of gears, toothed belts and pulleys, and the like, so that the platen is rotated synchronously with and at the same speed as the paper feeding rollers driven by the drive motor.
Some sheet feeding apparatuses in these printers can selectively feed a continuous form and cut sheets. More specifically, a continuous form having feed holes at both side edges thereof is fed to the printing position on the platen by way of a first paper path including pin tractors, while cut sheets are fed to the printing position by way of a second paper path located between the printing position and the first paper path. In these sheet feeding apparatuses, the feeding speed of the paper feeding rollers occasionally becomes higher than that of the pin tractors due to a small dimensional error, even though they are designed to feed a sheet at the same speed. Thus, a provision has been made to feed a continuous form positively by the pin tractors and to cause the continuous form to slip over the paper feeding rollers in case the feeding speed of the paper feeding rollers becomes higher than that of the pin tractors. To elaborate, when a continuous form is selected for printing, the paper feeding rollers on the first paper path are set apart from the sheet and the other paper feeding rollers are set to contact the sheet with weaker pressure. When cut sheets are selected for printing, on the other hand, the paper feeding rollers at both sides of the printing position must contact the sheet with pressure necessary to feed the cut sheets of a variety of thickness only by the friction between the paper feeding rollers and the sheets.
If the paper feeding rollers and the pin tractors are structured to be driven by a single driving source like a motor, cut sheets are not to be printed until the continuous form is removed from the pin tractors or the pin tractors are disconnected from the driving source so as not to feed the continuous form together with the cut sheets.
Therefore, these conventional printers have a problem that it requires two different operations, namely: to attach or detach the feeding rollers to or from their opposite ones; and to connect or disconnect the pin tractors with the driving source. If the continuous form is undesirably pressed by the paper feeding rollers at the front and rear of the printing position because of failure in adjusting the contact pressure of the paper feeding rollers, the pin feed holes at the both side edges of the continuous form will be torn by the pins of the pin tractors which move slower than the paper feeding rollers, or the cut sheets selected for printing can not be fed properly because they are not supported firmly between the paper feeding rollers at the front and rear of the printing position.
As for the platen, it is made of a cylindrical rubber so as to reduce the printing noise as much as possible. The inertia force of the rotating platen necessarily becomes great owing to the weight of the platen, which results in inaccuracy in rotating or stopping the platen in response to the operation of the drive motor. More specifically, because of the inertia rotation of the platen due to its inertia force, a sheet of printing paper is not always fed exactly for a determined amount.